1. Field of the Invention
This invention relates to a magnetic shielding material for preventing a strong magnetic field generated by a high magnetic field-generating source, such as a superconducting magnet, a normally conducting electromagnet, a superconducting solenoid, a normally conducting solenoid or a high-performance permanent magnet, from leaking to a space outside a predetermined space and avoiding bad influences of a magnetic field leaking to the outside.
2. Description of the Related Art
With recent advance of the magnetism-utilizing technique, the number of machines and apparatuses utilizing a strong magnetic field has been increasing. For example, an apparatus generating a strong magnetic field of 1,500 to 5,000 gauss is used for a magnetic resonance imaging apparatus (ordinarily called "MRI") developed as a medical tomographic means. The intensity of the used magnetic field is more increasing for enhancing the performance.
Furthermore, it is considered that a linear motor car, which is expected to be practically used in the future, will also become a source generating a strong field.
However, such a strong magnetic field causes erroneous operations or functional disturbances in electronic machines or apparatuses such as computers and magnetism-utilizing articles such as magnetic cards. Accordingly, the magnetic field must be cut off by some measure or other. However, the development of means of a high practical utility for cutting off such a strong magnetic field efficiently and assuredly is delayed at the present.
For example, for the above-mentioned MRI, a method is ordinarily adopted in which the magnetic field is cut off by attaching thick iron plates having a large thickness of approximately 2 cm to six faces, that is, four wall faces, the ceiling and the floor, of a chamber in which MRI is set. According to this method, it is possible to reduce the magnetic field of about 100 gauss on the wall face in the chamber to less than 5 gauss on the wall face outside the chamber.
However, this method in which the chamber is surrounded by thick iron plates has problems in that, since the weight reaches scores of tons, technical problems arise in connection with construction and working, and the construction expenses drastically increase.
A method in which a sheet of Permalloy (50Fe-50Ni) is used instead of the thick iron plate has been proposed. Permalloy has a higher magnetic permeability than that of the iron plate, but since the saturation magnetic flux density is low, the magnetization is easily saturated and Permalloy is not sufficiently advantageous over a mild steel sheet. Moreover, it has been found that in Permalloy, the magnetic permeability is remarkably reduced at points where a stress is imposed by processing or points where welding is carried out for connection, and the magnetic field leaks to the outside from these points. For this reason, it is not practical to use Permalloy as a shielding material for cutting off a strong magnetic field at the present.